A power supply operating in the Zero Voltage Switching (ZVS) and forward modes described in U.S. Pat. No. 5,877,946 issued Mar. 2, 1999, entitled A FORWARD CONVERTER WITH AN INDUCTOR COUPLED TO A TRANSFORMER WINDING, in the name of W. V. Fitzgerald (the Fitzgerald patent), includes a main switching transistor coupled to a primary winding of a main power transformer. Output supply voltages are developed from voltages developed in secondary windings of the transformer. When the transistor is conductive, a current pulse is developed in the primary winding of the transformer and in the transistor. A voltage is also developed in a current sensing resistor coupled in series with the transistor. The voltage in the current sensing resistor is coupled to a first input of a comparator of a control circuit. A second input of the comparator is coupled to a capacitor that develops a voltage varying in accordance with an output voltage of the power supply for providing regulation.
During a given conduction interval of the transistor, the comparator is triggered when the current sensing resistor voltage exceeds a threshold voltage of the comparator established by the capacitor voltage. Ann output of the comparator is coupled to the base of the transistor for controlling the turn off instant of the transistor on a current pulse-by-current pulse basis.
In normal operation, a voltage, present across the primary of the main power transformer, reduces the voltage across the supply inductance. This voltage is proportional to the output voltage produce in a given secondary winding of the transformer. The output voltage produces from the secondary winding is stepped up by the turns ratio of the transformer. When the switching transistor turns off at the end of each cycle, a negative voltage pulse, reflected from the secondary side of the transformer reduces the collector voltage of the transistor.
Excessive collector voltage may be developed in the main switching transistor if an overload condition occurs in one of the secondary winding. The over-voltage is caused by excessive circulating current in a resonant supply inductance which resonates with a resonate capacitor that are coupled to the collector of the main switching transistor to form ZVS.
If a severe overload occurs on one of the secondary windings, which causes the power supply to fall out of regulation, the voltage across the primary winding of the transformer also drops since the voltage reflected by the turns ratio of the transformer is reduced. The result is that the collector voltage of the transistor may become excessive.
When the output voltage produced from the secondary winding falls out of regulation, a maximum current limit is established by the control circuit. Under an overload condition, the transistor will still allow the maximum current to flow through the supply inductance. However, energy stored in the supply inductance is not delivered to the load through the transistor. The stored energy produces resonant current in the resonant current in the resonant capacitor when the transistor is turned off at the end of the cycle and causes the collector voltage of the transistor to rise substantially above the normal operating voltage, possibly exceeding the breakdown voltage rating of the transistor. Since, under overload, the energy that is stored in the supply inductance during each cycle is not delivered to the load, the energy returns back to the unregulated supply that energizes the transistor via a reverse or negative current. It may be desirable to reduce the resulting excessive collector voltage.